Field of the Invention
The invention relates to a post-tension cable protection system and a method for installing such a system to be used in the construction of high-rise buildings or any other post-tension reinforced concrete structure. The invention also relates to a method for remediation of a defective post-tension reinforcement system.
Description of the Related Art
In high-rise building construction, steel cables or tendons in plastic sheathing are positioned in forms and on decks before concrete is poured so as to run through the center of a post-tensioned concrete slab. Such slabs used in residential construction are typically 8 inches thick and use 3,000 to 10,000 psi concrete. The tendons are stressed after the concrete has attained a compressive strength of no less than 75% of its 28 day strength as confirmed by field cured test cylinders. Modern tendons have seven high-strength steel wires wound together and placed inside the plastic sheathing. One end of the slab has a post-tension or PT anchor embedded in the concrete. The other end of the slab has a PT anchor and a pocket formed in an edge of the slab. The pocket is formed by a plastic pocket former that is later removed and leaves an opening which the PT cable runs through. The opening made by the pocket former is large enough so that the cable end can be cut to the correct length and a grease cap can then be applied. The PT cable is stressed and must meet the 7% tolerance required for elongation. After stressing and removal of the plastic pocket former, the tendons ends are cut off within the pockets in which the anchors are located and the pockets are filled with grout to protect them from corrosion. PT concrete is superior in strength to steel-reinforced concrete which is not tensioned.
The problem which occurs is that even when using non-shrink grout, over time the grout will shrink and a gap will develop which allows moisture to enter. Human error in mixing and a need to retemper the grout mix over extended periods of time contributes to the problem. The moisture causes corrosion which can lead to failure, requiring remediation, especially near the ocean where moisture and salt content in the air are high. Conventional approaches to sealing the cavity have been unsuccessful which requires difficult and expensive remediation. Another problem associated with grouting is that traditional grouting methods do not bond the grout to the smooth concrete finish left by the pocket former. While traditional methodology calls for roughing the pocket surface with a wire brush, a wire brush has little to no impact on concrete stronger than 2000 PSI. When there is no bond, water ingression is likely. Wet packing of grout to a hardened and contaminated concrete surface is also very problematic.
U.S. Pat. No. 4,719,658 to Kriofske discloses the sealing of an anchor in post-tension systems and specifically states that it is preferable if the grease cap does not fit snug with the anchor. However, according to current building codes, a grease cap must be properly seated within the base plate. The distance from the end of a properly seated grease cap to the face of the slab is exactly 1 inch. Current building codes call for a 1 inch minimum coverage over the grease cap to ensure that the cable is protected in aggressive environments. If the grease cap is not properly seated, which is noted as preferable in Kriofske's patent, the 1″ minimum requirements cannot be achieved and a PT plug would not be able to fit the cavity since it is configured to fit only when a grease cap is properly seated. Furthermore, if the grease cap is not properly seated into the anchor and grease is pumped into the cavity, the excess grease will escape from under the cap and enter the rest of the cavity. A patch cannot be made in the presence of grease which is a bond breaker and must be completely removed. Using a wire brush as noted in traditional methods does not remove grease nor does it roughen the surface.
U.S. Pat. No. 3,639,555 to Steffan teaches a method of making a concrete plug. The plugs are used for architectural finishing and are therefore only provided for aesthetic purposes. The use of concrete for patch material in post-tension systems is specifically prohibited by code in substantially every jurisdiction because it shrinks too much to provide even minimal protection from moisture intrusion. The combination of metallic traces and concrete's relatively high levels of shrinkage make it a very poor and prohibited patch material for cable ends.
U.S. Pat. No. 4,502,554 to Jones teaches an expansible tool for reaming undercuts in cylindrical holes in order to place bolts in the holes. The reaming tool of Jones requires flooding the cavity with water and other fluids to flush out concrete cuttings and cool the cutting tool during the reaming process. However, the use of water would promote corrosion in post-tension systems which is exactly what a grease cap and a patch are supposed to prevent. The addition of water or any other fluid to a Post Tension Cable Systems is strictly forbidden.